The invention relates generally to lasers and particularly to diode-pumped intracavity doubled lasers.
Intracavity frequency doubling of solid-state lasers has been plagued by the so-called "green problem" as described in T. Baer J. Opt. Soc. Am. 3 (1175) 1986, which causes the doubled output to exhibit chaotic fluctuations in power.
Stable intracavity frequency doubling has generally been achieved by some researchers only by forcing the laser to operate in a single longitudinal and single polarization mode. However, others have recently reported stable operation in two longitudinal modes of orthogonal polarizations, when a doubling crystal, which is birefringent, is made to act as a waveplate and a second waveplate is used to decouple the two orthogonally polarized longitudinal modes. An intracavity "noise suppression" etalon is used in these latter laser constructions to narrow the longitudinal mode spectrum as is known, for example, from L. Y. Liu, et al., Opt. Lett., Vol. 19 (1994) pp. 189; and shown in U.S. Pat. No. 4,656,635 of Baer & Kierstead.
There is an optimum polarization for frequency doubling, defined by the orientation of the doubling crystal's axes, such that in this optimum orientation, the doubling efficiency is maximized. Constructions seeking to optimize doubling in this manner (as in Baer, et al., U.S. Pat. Nos.: 4,653,056; 4,723,257; 4,756,003; 4,701,929; 4,872,177), require the intracavity fundamental light to be polarized, so as to match the optimum polarization for the doubling crystal.
When the doubling crystal is a birefringent one, such as KTP, there are additional problems caused by depolarization of the fundamental laser radiation, as noted in papers of Liu, et al., Nightingale & Johnson, and James, et al. These problems may be avoided by cutting the length of the doubling crystal to act as a waveplate with an integral number of waves retardation, i.e., to act as a full-wave plate; see for example, J. L. Nightingale & J. K. Johnson. 0.6 W Stable, Single-Frequency Green Laser, Proceedings of Topical Meeting on Compact Blue-Green Lasers, Feb. 10-11, Salt Lake City, Utah (1994), paper PD6-1. In addition, the frequency doubler must be actively temperature stabilized, otherwise the birefringence will vary with temperature shifts (Nightingale & Johnson, Liu, et al.). Nightingale & Johnson use a ring cavity to achieve single-frequency operation and this requires tight temperature control of the laser crystal and of the doubler. Liu, et al., use a folded cavity with an intracavity etalon to achieve single-frequency operation and this also requires tight temperature control. Oka & Kubota have shown that an intracavity waveplate can be used to obtain stable frequency doubling of unpolarized light, provided the doubler and the waveplate are actively temperature stabilized to within a fraction of a degree. Thus, all of these constructions require additional control or light conditioning elements, and in general result in relatively low overall electrical-to-light output efficiency.
To increase output power of the second harmonic, one prior art construction has the cavity folded to achieve two-pass doubling, as in U.S. Pat. No. 4,887,270 of Austin. One cannot, in general, simply extract the backward pass through the back mirror, after it has passed through the laser rod, because the majority of solid-state media are strongly absorbing at their second harmonic wavelength.
If the laser crystal is an anisotropic medium such as Nd:YAG, then the folding mirror will cause a slight discrimination between the gains of s- and p-polarizations of laser emission. The s-polarization experiences the highest mirror reflectivity, and, therefore, the lowest loss in the laser cavity. Lasers tend to lase in their lowest loss state, so the polarization of the fundamental orients itself to minimize the loss; i.e., the laser lases in s-polarization.
When the axes of the doublers are aligned with the laser polarization, the frequency doubling and the subsequent output coupling from the cavity of the fundamental light, is yet another loss mechanism for the laser. When the laser experiences this loss, it tends to lase in a different polarization to minimize loss. Thus, an inherently unstable situation occurs. As a result, folding the laser cavity without employing strong polarizing techniques, such as an intracavity Brewster plate to inhibit lasing in other polarizations, results in lower output power.
In most materials, especially Nd:YAG, there is strong thermal birefringence which rotates the fundamental polarization. When a birefringent doubling crystal is placed within the cavity, it too rotates the polarization of the intracavity light. Such rotation causes loss by output coupling through the strong polarizing element; e.g., reflection off the Brewster plate and out of the laser cavity.
Nevertheless, folded intracavity doubled lasers have been made by controlling the birefringence of the frequency doubling crystal with precise temperature control, and compensating for the thermally induced depolarization introduced by the laser crystal, as reported in a 1988 paper of Oka and Kubota; in Liu, et al., supra.; in Anthon & Sipes, Proceedings of Conference on Lasers and Electrooptics, Anaheim, Calif., paper CWC3 (1990); and in U.S. Pat. Nos. 4,887,270 and 4,884,277. These techniques complicate the construction of the laser and require a high degree of thermal control.
Microchip lasers have been made to achieve single-frequency operation by making the laser cavity so short that only one longitudinal mode can lase. Effectively, this requires cavity lengths less than one millimeter, as reported by Taira, et al. in Opt. Lett. Vol. 16 (1991) pp. 1955. To lase effectively with such short cavity lengths, the laser crystals, which are pumped longitudinally, are to exhibit extremely high absorption of the diode pump light. MacKinnon, et al. in Proceedings of Conference on Lasers and Electrooptics CLEO '94, May 8-13, Anaheim, Calif. (1994) have recently reported using these lasers in frequency doubling constructions with low amplitude noise by bonding a doubling crystal directly onto the laser chip, thus retaining a short cavity length.
Another construction for a high absorption laser uses an ultra-short length of gain medium placed adjacent to one cavity mirror. This latter construction was developed to prevent spatial hole burning. Since all longitudinal modes have a node at the end mirrors, no hole burning can occur in a gain medium at an end mirror if the gain medium is sufficiently short. Reportedly, the gain length should be shorter than 250 .mu.m. G. Kintz & T. Baer IEEE J. Quantum Electron, Vol. 26 (1990) pp. 1457.
In summary, prior constructions employ active temperature stabilization of the frequency doubling crystal, of intracavity waveplates or etalons and, generally, of the laser crystal itself. Slight changes in birefringence, due for example to misalignment of the doubling crystal, cause output instabilities. The lasers are forced to operate in a single longitudinal mode in one or both of two orthogonally polarized directions.
A passively stabilized laser that does not require tight temperature control or many intracavity elements is desirable. Preferably, a construction for such a laser is to be robust and insensitive to vibration.
A linear cavity is deemed simpler and cheaper to construct because it requires fewer optical elements, and potentially results in fewer losses than a folded cavity. A further objective is a stable operating construction that recovers the doubled-frequency light generated on the return pass back through the doubling crystal, and directs it into a usable output beam.